Kallistatin, a serine proteinase inhibitor (serpin), is expressed in the endothelial and smooth muscle cells of blood vessels. The potential function of kallistatin in vascular biology was investigated by studying its role in the proliferation and migration of cultured primary aortic vascular smooth muscle cells (VSMCs) in vitro and in neointima formation in rat artery after balloon angioplasty in vivo. Exogenous kallistatin induced a >2-fold increase of VSMC proliferation and cell growth as measured by [(3)H]thymidine incorporation and cell counts and a 2.3-fold increase of cell migration in modified Boyden chambers. In balloon-injured vessels, endogenous kallistatin mRNA and protein levels increased up to 10-fold as determined by competitive polymerase chain reaction and by ELISA. Intense staining of kallistatin mRNA was identified in the proliferating VSMCs of balloon-injured arteries during cell migration from media to neointima by in situ hybridization histochemistry and immunohistochemistry. We observed an induction of kallistatin expression by platelet-derived growth factor (PDGF) and upregulation of p42/44 mitogen-activated protein kinase (MAPK) activity by kallistatin in cultured VSMCs. Conversely, adenovirus-mediated transfer of kallistatin antisense cDNA into cultured VSMCs inhibited PDGF-induced p42/44 MAPK activity and cell proliferation. Furthermore, local delivery of adenovirus carrying kallistatin antisense cDNA significantly downregulated kallistatin mRNA levels and attenuated neointima formation in balloon-injured rat arteries in vivo. These results indicate that kallistatin may play an important role in mediating PDGF-induced MAPK pathway on VSMC proliferation and in neointima formation after balloon angioplasty.